In an erasable and rewriteable Digital Video Disk (DVD), it is desirable to write a new block of data over existing data such that a data clock inherent in the new data block accurately conforms in both frequency and phase to the data clock inherent in the old data block. In the prior art, placement of data to be written on a recording layer of a rewriteable optical disk is typically determined by including synchronization information between fixed-length data sectors. Sectors are repeating units of pre-determined length. FIG. 1A shows a plan-view of a prior art optical disk 10 in which data stored along a servo track 12 is divided into sectors 14. FIG. 1B shows an expanded view of a sector 14 of the optical disk of FIG. 1A. The sector 14 includes a header 16, a data field 18 having a predetermined length, and an edit gap 20.
FIG. 1C shows an expanded view of the header 16 of FIG. 1B. The header 16 includes synchronization information 22 and track address information 24. The synchronization information 22 is also referred to as the sync field. The synchronization information 22 is permanently encoded on the recording layer of the optical disk 10 within the sectors 14. Data written onto the recording layer of the optical disk is synchronized to a write clock. The write clock is synchronized to a reference clock signal which is generated periodically as the synchronization information passes by an optical transducer while the optical disk is rotated. The reference clock signal provides information regarding the position of the optical transducer with respect to the synchronization information. However, while data within data field 18 is being written by the optical transducer, the reference clock signal may drift in frequency and phase. That is, when the optical transducer is between points at which synchronization information exists, the frequency and phase of the write clock can drift with respect to the synchronization information located within sectors. Drift of the write clock with respect to the synchronization information can be caused by disk rotation speed variations, servo track eccentricity and the cumulative effect of other variations in an optical disk recorder such as clock frequency drift. In general, the greater the distance between sync fields, the greater the drift of the write clock.
The edit gap 20 shown in FIG. 1B is included within the sector 14. A data field 18, which includes a fixed number of data bits, is typically written to the sector 14 of the recording layer of the optical disk 10. The edit gap 20 accommodates variations in the placement of the last data bit of the data field which is written to the sector. That is, although all data fields normally contain the same number of data bits, the edit gap allows the placement of the last data bit of a data field to be different each time the data field is rewritten. Therefore, placement of data bits written to the recording layer is not required to be as precise as the placement would be required to be if the edit gap did not exist. Edit gaps are needed to accommodate for drift of the write clock in prior art rewriteable optical disks.
Presently existing DVD-ROM formats do not include physical sectoring of data stored on the recording layer of an optical disk. Therefore, synchronization fields and edit gaps are not provided. When reading a ROM optical disk, a read clock is produced from the data stored on the optical disk, and therefore, no synchronization information is required.
The DVD-ROM format specification organizes data into fixed-length data fields for error correction code (ECC) purposes. Each data field has an associated header containing synchronization and address information to facilitate data readout. The synchronization and address information is stored on the disk in the form of data pits which are indistinguishable from the data pits used to encode user data. Although a DVD-ROM data field, together with its associated header information, makes up a "physical sector" for the purpose of a read-only memory, it does not satisfy the requirements of a physical sector for the purposes of a rewriteable optical disk memory. For this reason, all sectoring of the DVD-ROM format is treated as "logical sectoring." A logical sector is contained within the data, whereas a physical sector contains the data. Therefore, all synchronization information, addressing and other DVD-ROM formatting are treated in the same manner as user data, and are written on the disk in the form of data marks at the same time user data is written.
As a result, writing data to the recording layer of a rewriteable optical disk which is compatible with DVD-ROM formats requires data to be written to a disk having no physical sectors on the unwritten disk, and consequently, no address or synchronization information in dedicated areas within such physical sectors. Furthermore, edit gaps cannot be included. Without edit gaps, the data marks must be written with sub-bit accuracy during overwriting of pre-existing data.
The DVD-ROM format specification also describes a constant linear velocity (CLV) disk format which maximizes storage capacity but requires a continuously varying spindle speed. In developing an erasable DVD, performance can be significantly improved by radially dividing the disk into constant angular velocity (CAV) zones. Short length seek operations on a zoned CAV disk can be completed very quickly, because there is no need to change spindle speed as long as the seek does not cross a zone boundary. Since most seeks in a disk drive are short in length, zoned CAV provides much shorter average seek times than CLV, with essentially no loss of capacity. In addition, standard DVD-ROM drives can read zoned CAV disks because the fractional change in clock frequency across a zone boundary is only about 1%. In a DVD-ROM drive, the clock frequency change is within the acquisition range of the spindle speed servo loop and the data readout clock's phase-locked loop.
As a result, it is desirable to have a rewriteable optical disk, and optical disk recorder capable of recording data on the optical disk, wherein the recorded disk is compatible with DVD-ROM standard formats, and is readable by a standard DVD reader, and wherein pre-existing data on the optical disk can be rewritten (sometimes called over-written) with new data with sub-bit accuracy. The optical disk and optical disk recorder must be capable of generating a write clock which is synchronized with sub-bit accuracy to absolute positions along the servo tracks of the optical disk. Further, it is desirable to be able to write using standard DVD data formats.